1. Industrial Field of the Invention
The present invention relates to a high-strength and low-thermal-expansion alloy employed for high-accuracy mechanical parts which may be deteriorated due to elevation of temperature in use, heat resistant core wires of a low sag grade for power-transmission lines or the like. It further relates to a high-strength and low-thermal-expansion wire made of the alloy, and to a method of manufacturing the alloy wire.
2. Prior Art
Conventionally, an aluminum wire strand having a steel core (referred to as ACSR wire) has been utilized as an overhead power-transmission line. Due to an increase in power demand and the rise in land prices in recent years, however, there has been an increasing demand for a core wire having a high strength and a low coefficient of thermal expansion in place of the conventional aluminum wire strand having a steel core. When a high-strength and low-thermal-expansion wire is utilized as a core material of an aluminum wire strand, a plurality of material wires must be bundled. In order to evaluate performances of the wire bundling or twisting working, there is conducted a torsion test in which one end of a combination of material wires is fixed and the other end is twisted, so that the performances will be evaluated by torsional values.
Fe--Ni system alloys for this purpose are disclosed, for example, in JP-B2-56-45990, JP-A-55-41928, JP-B2-57-17942, JP-A-55-122855, JP-A-55-128565, JP-A-55-131155, JP-A-56-142851, JP-A-57-26144, JP-A-58-11767 and JP-A-58-11768, which have been suggested to improve strength and torsional property. Further, in order to improve strength and torsional property of those alloys, there have been suggested high-strength and low-thermal-expansion alloy wires (ACIR wires) and manufacturing methods of alloy wires disclosed in JP-B2-60-34613, JP-B2-2-15606, JP-B2-2-41577 and JP-B2-2-55495.
Any of the above-mentioned conventional high-strength and low-thermal-expansion alloy has a composition of Ni, or Ni and Co in a range of 35 to 50%, interstitial type solid-solution strengthening elements such as C (carbon) and N (nitrogen), several kinds of substitution type solid-solution strengthening elements such as Cr and Mo, and several kinds of precipitation hardening elements such as Ti and Nb in such a range as not to deteriorate the low thermal expansion property, and balance of Fe. As subjected to solutioning heat treatment or annealing heat treatment, a favorable torsional property can be obtained from any of the alloys, but tensile strength is, at most, in a range of 50 to 80 kgf/mm.sup.2. In such a state, the alloys are unsuitable for use as a core wire of a low sag grade for an overhead power-transmission line. Any of the alloys has a work hardening capacity is larger than those of conventional low-thermal-expansion alloys such as a 36% Ni--Fe alloy and a 42% Ni--Fe alloy, and a tensile strength of 100 to 130 kgf/mm.sup.2 can be obtained by cold working. Consequently, alloys have been partially put into actual use.
However, most of piano wires employed as core wires of the conventional steel-core aluminum wire strands have strength of 170 kgf/mm.sup.2 grade, and low-thermal-expansion alloy wires having tensile strength which is substantially the same as or close to that of the piano wires of 170 kgf/mm.sup.2 grade have been required for increasing transmission capacities of the power-transmission lines without rebuilding pylons. Moreover, if the conventional high-strength low-thermal-expansion alloy wires mentioned above are simply subjected to high reduction working in the cold temperature zone, their torsional property which expresses a deforming capacity with respect to torsion will be largely deteriorated. Therefore, not only the above-mentioned alloys but also various kinds of complicated manufacturing methods have been proposed to make tensile strength and torsional property compatible with each other.
For example, in JP-B2-60-34613 and JP-B2-2-15606, stress relieving annealing is performed at a stage prior to cold working or in the course of cold working in order to attain compatibility of tensile strength and torsional property. Those manufacturing methods discloses that favorable torsional property can be obtained when deformation of the surface caused by scalping is removed by annealing heat treatment.
On the other hand, although JP-B2-2-41577 and JP-B2-2-55495 disclose alloy wires manufactured in substantially the same process as the above-mentioned JP-B2-60-34613 and JP-B2-2-15606, it is stated that carbide of Mo.sub.2 C generated during annealing after cold working contributes to improvements of strength and torsional properties. However, one of the inventors of JP-B2-2-41577 and JP-B2-2-55495 mentions an improvement of torsional property in a report titled "Effect of processes of drawing on torsional property of high-tensile strength Invar alloy wire" (Wire Journal International vol. 21. No. 4 (1988), p. 84). The report says that torsional property can not be fully improved merely by performing annealing heat treatment for precipitated Mo.sub.2 C after cold working, and that a drawing angle of a die must be decreased so that, particularly in a distribution of hardness in a cross section of alloy wire after drawing, hardness in the center will be the highest, and a special jig called Christopherson tube for enhancing lubricative property is required.
However, the improvement of torsional property by decreasing a drawing angle of a die and using the jig results in an increase in the number of drawing passes (a ratio of a decrease in a cross-sectional area per pass can not be increased when the drawing angle is decreased). Also, it takes time to change the process in the manufacturing line. Consequently, this manufacturing method is extremely inefficient for manufacturing an alloy wire having an entire length of several kilometers.
Taking the above problems into account, the present invention has been proposed.